EEK150 Project 1

Making of:PD Data Acqusition System

Author:Johannes AvahedenVishal Mathur

Supervisor:Xiangrong Chen

Department of Electric Power EngineeringChalmers University of technologyGothenburg, October 27, 2013

Abstract

Partial discharges (PD) occurring in insulation systems can becharacterized by the magnitude of the apparent charge as well as thephase angle relative to the power line cycle. This project describesa partial discharge measurement system based on USB digitizer NI-5133, which is a 2 channel high speed digitizer with the analysis andacquisition program written in labVIEW. Once the analog data hasbeen acquired it is processed and then presented grafically on a phaseresolved basis. The acquisition system was developed using artificiallygenerated pulses from a signal generator. This setup was subjectedto actual high voltage test along with the haefley 568 PD system forcomparison.

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Acknowledgements

We would like to extend much gratitude to our supervisor:Xiangrong Chen

who have been of great help throughout the project!

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Contents

1 Background 1

2 Aim of the project 1

3 Task and problem description 23.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23.2 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

4 The project scope 3

5 Theory behind PD-measurement 45.0.1 Phase resolved analysis . . . . . . . . . . . . . . . . . . 5

6 Method 66.1 Choice of Hardware . . . . . . . . . . . . . . . . . . . . . . . . 66.2 Quadripole synchronizing port voltage test . . . . . . . . . . . 76.3 Software development . . . . . . . . . . . . . . . . . . . . . . . 9

6.3.1 Choise of development platform . . . . . . . . . . . . . 96.3.2 Creation of a basic software structure . . . . . . . . . . 9

6.4 First prototype . . . . . . . . . . . . . . . . . . . . . . . . . . 106.4.1 Block part 1 . . . . . . . . . . . . . . . . . . . . . . . . 106.4.2 Block part 2 and 3 . . . . . . . . . . . . . . . . . . . . 126.4.3 Block part 4 . . . . . . . . . . . . . . . . . . . . . . . . 12

6.5 Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . 136.5.1 Initial signal processing . . . . . . . . . . . . . . . . . . 136.5.2 PD-signal processing . . . . . . . . . . . . . . . . . . . 14

6.6 Final software version . . . . . . . . . . . . . . . . . . . . . . . 16

7 Results 18

8 Discussion 218.1 Future additional work . . . . . . . . . . . . . . . . . . . . . . 21

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1 Background

Measurement and analysis of partial discharge (PD) is an essential tool fordiagnosis of insulation condition [1]. As the High voltage devices ages, theinsulation tends to become weak which could lead to breakdowns due tovarious phenomenon occurring inside the device, raising the risk of insulationfailure, which could be disastrous. Thus it becomes imperative to have asystem that enables analysis of potential insulation problems so that thedevices may be repaired or replaced well in advance.

To analyze the PD’s the magnitude of discharge is measured. Furthermorethe phase position (ϕ) when the discharge takes place during the sinosoidalvoltage period is measured. When the PD magnitude (q) is plotted againstthe phase angle the PD pattern is obtained. This pattern is what characterisea certain type of partial discharge. Traditionally this is done with an analogPD pattern analyzer but as most modern systems today usually run digitally,a way to analyze PD patterns digitally is therefore desirable.

2 Aim of the project

The Aim of the project is to develop a portable, digital system for partialdischarge acquisition using an analogue input from hardware such as HaefelyPD568.

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3 Task and problem description

The task of this project is to develop a digital Partial discharge detection sys-tem for a sinusoidal voltage application. The system shall be able to measurepartial discharges and extract the parameters ϕ and q of the discharges.

The problem could be divided into two parts, choice of hardware anddevelopment of software.

3.1 Hardware

A research has to be carried out for selecting the proper hardware. Thishardware is required to have a high bandwidth for better resolution and ahigh frequency, multichannel interface.

3.2 Software

Software compatible with this device will be used for programming as wellas displaying the graphical outcome of the partial discharge using the phaseresolving technique. The data thus collected should be in a position to besaved to disk for further external analysis.

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4 The project scope

The main focus will be upon establishing the link between the host computerand the PD detector, that shall include hardware selection, interfacing as wellas software selection and programming. Further analysis of the discretizeddata shall be done to find out two parameters crucial for phase resolvedtechnique i.e. peak magnitude as well as the phase angle with respect to thepower voltage frequency.

However other parameters used for PD characterization such as averagevalue of integrated distribution parameters, would not be included. Analysisof the complete setup calls for a Digital signal processing approach whichshall not be discussed as it is not within the scope. All the analysis anddiscussion would be based on a phase resolved pattern Analysis only.

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5 Theory behind PD-measurement

A number of methods have been developed for detection of phenomenon thatoccurrs during PD, such as the electromagnetic field, dielectric losses and theelectrical method. This project is based on the electrical method.

The electrical detection method is based on the measurement of the cur-rent that flows out from the terminals of the object under test due to theoccurrence of the internal discharges. The method uses the measurementcircuit as shown in Figure 1.

Figure 1: The test system

U - high-voltage supplyΩ - CalibratorCx - Test objectCc - Coupling capacitorQ.pl - Quadripole

The test circuit consists of a power source, the object under test Cx, thecoupling capacitor Cc along with a quadripole. The current signal originatingby the partial discharges is converted into a voltage signal by the Quadripole.This is measured by the oscilloscope.

The quadripole impedance is a determining factor on the pass band of themeasuring system. Systems based on phase resolved analysis almost alwaysuse the impedance consisting of resistor and inductor in parallel, forming aresonating circuit inside a quadripole. This in turn has a very low pass band,allowing only the signal of interest to pass by and offering high impedance tothe noise signals. The phase resolved analysis is the choice for data acquisi-tion system in most cases.

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5.0.1 Phase resolved analysis

Analysis based on phase resolved technique depicts the type of defect andthe level of insulation degradation reached. This is done with the help ofanalyzing the discharge occurrence pattern over the power line sinusoidalwave. This type of distribution is known as the ”fingerprint of a discharge”.This plays an important role in identifying the kind of defect present ininsulation and the level of deterioration. A typical PD-pattern is presentedin Figure 2.

Figure 2: A typical PD-pattern recieved with the phase resolve method [1]

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6 Method

The development was divided into two parts. First proper hardware were tobe chosen. Second the software to run the hardware were to be developed.

6.1 Choice of Hardware

As explained in Section 3, the digital PD-acquisition system needs to havethe capability to sample and store data to characterize PD’s. It should alsobe capable of transferring the data obtained to the host PC’s random accessmemory to generate (ϕ, q) plots.

NI USB- 5133 digitizer fulfills all the needs required for this system.

Figure 3: The NI USB-5133 digitizer [2]

The main factors that were considered while selecting digitizer were thesampling rate, which for the USB-5133 is 100 MS/s. That assures a truthfulreproduction of the analog signal to digital. The bandwidth of 50 MHz and2 simultaneously sampled channels with 8-bit resolution makes it an idealchoice. It is having 8 Mb memory equally divided between two channelsCH1 and CH2. The bus powered, plug and play feature of this device makesit portable and easy to use.

As presented in Figure 1, the USB digitizer would be connected to the thesynchronizing port of the quadripole via channel CH0 or CH1. The digitizeris having an operating voltage range of 40 Vp−p, i.e. from -20V to +20V. In

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order to ensure that the input voltage to the digitizer does not exceed thegiven limit, the voltage profile of the Haefley PD 568 should be known inadvance. For this reason a test was conducted in a high voltage laboratoryto determine the input – output voltage characteristics of the quadripole.

6.2 Quadripole synchronizing port voltage test

The laboratory Setup for finding the I/O voltage relation of the QuadripoleSynchronizing port is presented in Figure 3

Figure 4: The test setup for the High voltage test

As presented in Figure 4, the test setup consisted of a high voltage testbench, that is connected to a coupling capacitor of 1nF value. The PDchannel of the quadripole was then connected to the PD detector system. TheSynchronizing channel was connected to a oscilloscope. Different values of theoutput voltage from the quadripole synchronization channel were recordedfor corresponding high voltage input from the high voltage test bench. Thevalues recorded is presented in Table 1.

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Table 1: Result of the high voltage test

Input voltage (kV rms) Output voltage (V peak- peak)

1.8 0.583.5 1.286 2.2510 3.815 5.620 7.625 9.6

The graph was plotted in matlab, see Figure 5, and corresponding slopeof the line was obtained where slope m= 0.385 and the constant b= -0.0922.These values describe the characteristic nature of the Syncronisation channel.With further extrapolation the output voltage for any given input voltage canbe calculated.

Figure 5: Synchronization voltage characteristics

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6.3 Software development

The software that were to be developed had to be able to handle mainly threetasks:

• Display the PD pattern• Extract the main parameters phase ϕ and the aparent charge q.• Gather and export the main parameters to a database.

These tasks were therefore set as requirements for the final software.

6.3.1 Choise of development platform

LabView is capable of handling all of the sampling cards features and there-fore suitable for the development of the end user software. As LabView alsowas suggested by the project client LabView was chosen to be the develop-ment platform.

6.3.2 Creation of a basic software structure

To be able to develop a basic software structure a development setup wasrealised. This setup if presented in Figure 6.

Figure 6: The Setup for initial software development

For the initial part of the software development a Wavetek 10MHz DDSFunction Generator model 29 was used to generate a synthesised sinusoidalsynchronisation voltage of ±3V at 50Hz. This signal was then connected tochannel 0 (CH0) on the USB-5133.As a reference the generated signal was

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sent to a Tektronix TDS 2004B Digital Oscilloscope, this to be able to seethat the digitalised signal presented in the developed software was correct.A PD-calibrator was then connected to CH1 on the USB-5133.

As the main structure for the software, the LabView function tree NI-Scope was used.For understanding of the NI-Scope functions the predefinedLabView VI NI-Scope Express was studied together with the NI-Scope doc-umentation in LabView.

6.4 First prototype

By following the basic NI-scope scheme and adding some features using atrial and error method the first prototype was created. This is presented inFigure 7 and 8.

Figure 8: The visual interface

6.4.1 Block part 1

The first part consist of four main blocks. First a controller is added (herenamed ”resource name”) where the user will be able to choose what hardwarethey want to initialise.

The second block takes the named hardware and initialise that. This

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block then generates two outputs that will follow throughout the whole pro-gram, instrument handle and error.

The third block is the auto setup that will do a rough setup of the sam-pling card, and takes care of any future unattended parameter.

The forth block is the Horizontal setup. Here real-time acquisition isenforced and since realtime is used only one record is needed. Furthermorethe sampling rate aswell as the time reference position is set. This will ensurethat the maximum sampling rate of the card is used and that the presentedwave-graph begins at time t=0s. The min record length dictates how manysamples is recorded and later shown in the graph window, here the numberof samples for one period is chosen.

6.4.2 Block part 2 and 3

The First two blocks in the second part both define the vertical setup. Asthey are closely intertwined and different setups are to be set for the two inputchannels, a memory cluster was created (block part 3) to hold informationfor both channels.

For correct measurements the input impedance is stated, together withthe vertical coupling used. The desired vertical range (in volt) is chosen aswell as the offset and probe attenuation.

The third block in part 2 is the trigger controller. Here visual controllersis created so the user can choose positive or negative trigger slope, the triggersource channel and the trigger level (in volt).

The last block in part 2 is the initialisation of the acquisition.

6.4.3 Block part 4

In the Fourth part a while-loop is created for the actual reading. Here itsstated that both channel 0 and 1 are to be read. The timestamp type is acontroller that is to be set to ”relative” in the visual interface for a correctdisplay, this will keep the reference point steady at t=0s. The waveforminformation and the actual waveforms are then sent to their respectivelywindows in the visual interface for display.

When the stop button is pressed the program will jump out of the while-loop and the second to last block will terminate the acquisition and reset thehardware. When that is done the eventual error is sent for display in thevisual interface.

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6.5 Data processing

The next step of the software development was to process the acquired data.As explained in Section 5, to recieve the PD-pattern the parameters angleof the discharge ϕ and the amplitude of the discharge q has to be extractedfrom the sampled signal.

To creat a better overview of the softwares block scheme the Block part1 to 3 was made into a sub-VI. The dataprocessing was then performed inthe main-VI while loop of Block part 4. The data processing is then madein two steps. The main-VI is presented in Figure 9 where the data processsu-VI is presented as a green box.

Figure 9: The main-VI

6.5.1 Initial signal processing

Instead of using one signal reader, two was used. This to enable easier ma-nipulation of the two input channels CH0 and CH1 by themselves. The firstreader manages the PD-signal from CH1 and the second reader manages thesyncronisation voltage. The PD-signal is converted from a waveform signalto a dynamic signal and sent to the sub-VI Data Processing. The Synchro-nisation voltage is normalised to ±3V and merged with the PD-signal. Themerged signal is then sent to the graph window.

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6.5.2 PD-signal processing

The Data process sub-VI have six inputs; the PD-signal, threshold level,”ampification × 1”, ”ampification × 10”, minimum samplel length and dis-charge length. The sample length was set to 2MS. The other factors wereconnected to controls for easy calibration from the software interphase.

The chosen amplification is applied before the magnitude of the PD-signalis measured and extracted as a double. The phase at the discharge is thenextracted and these two parameters is sent back to the main-VI. The dataprocess sub-VI is presented in Figure 10.

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6.6 Final software version

For the final version a number of major changes was made. First, the graphi-cal inteface was change so that is contained two main parts, a graph present-ing an untampered signal was placed to the left and a graph presenting thePD-pattern to the right, see Figure 11. The PD-pattern graf was connectedto the data process sub-VI and to a signal generator that creates a sinu-soidial wave of one period with an amplitude two times the highest PD. Thisto present a perspicuous PD-pattern. With this the scaling of the presentedsynchronisation voltage was deleted, although the ability to scale the inputPD-signal was kept, if disabled. The only change of the PD-signal is that theabsolute value is used for the data processing, see Figure 12.

Figure 11: The final interface

Another major addition to the software is the ability to save the recordedPD data. During a session the Phase and Amplitude of the PD is recordedand upon pressing the ”Stop and Capture” button the session is ended andthe user is presented with the chiose of save this data to a file of chose onany available medium.

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7 Results

The final software where tested by performing a high voltage test on a shortHV-cable that was prepared so that it would trigger partial discharge easily.The Actual setup for the test is presented in Figure 13 and 14. The setupwas made in the same way as presented in Section 5, with the diffrence thatfor the final testing the Digital system was connected in parallel with theAnalog system, Haefely Type 561, for comparison, see Figure 14.

Figure 13: Test setup for the final testing

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Figure 14: Test setup for the final testing

The software cas calibrated using a Haefely KAL 451 set to producing100pC. The software was then set to present a correct value, See Figure 15.

Figure 15: Calibration of the software

The test object was subjected to voltages from 0 to 13,2kV. Pictures ofwhat the software presented at 0kV, 8,8kV and 13,2kV togeather with thecorresponding analog presentation is presented in Figure 16,17 and 18.

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Figure 16: Capture at 0kV

Figure 17: Capture at 8,8kV

Figure 18: Capture at 13,2kV

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8 Discussion

The Idea behind the software was to creat an interface that presented boththe input signal as would an oscilloscope and the PD-pattern. The softwaremostly worked as intended and gave a fairly acqurate presenmmtation ofthe PD-signal and PD-pattern, but there is one problem that we want tostress. The digital signal presented by the software appears to be flipped 180degrees. This can be observed by inspecting Figure 19. The reason for thisis unknown as it was discovered only after the testing had been done whenthe result was analysed. Further testing should be carried out in order todetermain the exact reason. But we believe the cause is the set trigger, asolution could then be to simply set the trigger at negative slope and invertthe persented reference voltage.

Figure 19: Comparison of digital and analog signals

8.1 Future additional work

One of the main benefits of a digital system is that it makes remote workpossible. Further development of the acquisition system could therefore prefe-ably involve to create a web-interface for remote access where the measure-ments could be saved in a database.

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References

[1] J.A. Ardila-Rey et. al. A Partial Discharges Acquisition and StatisticalAnalysis Software. 2012.

[2] url: http://sine.ni.com/nips/cds/view/p/lang/sv/nid/203720.

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